Algo como isso deve fazer o truque:
#The cnames are not needed, but nice to have. It will allow you to access mail via mail.domain.com
mail IN CNAME ghs.google.com.
calendar IN CNAME ghs.google.com.
docs IN CNAME ghs.google.com.
sites IN CNAME ghs.google.com.
start IN CNAME ghs.google.com.
#This is important
@ IN MX 1 aspmx.l.google.com.
@ IN MX 3 alt1.aspmx.l.google.com.
@ IN MX 3 alt2.aspmx.l.google.com.
@ IN MX 5 aspmx2.googlemail.com.
@ IN MX 5 aspmx3.googlemail.com.
@ IN MX 5 aspmx4.googlemail.com.
@ IN MX 5 aspmx5.googlemail.com.
IN TXT v=spf1 include:_spf.google.com ~all